1. Field of the Invention
The present invention relates to a power converter, or more in particular to a power converter comprising a current-type inverter adapted to prevent the high harmonic components of the output of the current-type inverter from having an effect on the load while adding various new functions to a power conversion system.
2. Description of Related Art
There has recently been suggested a PWM current-type inverter having an advantage in that the control circuit thereof is simple in configuration and that the noises of the motor driven by use of it are reduced, as disclosed in JP-A-62-163579. Also, an application of this PWM current-type inverter to the elevator control requiring high reliability and quietude is reported in Article No. 15 at 1987 National Conference of the Institute of Electrical Engineers of Japan.
The power converter of this type comprises a converter unit and an inverter unit with a DC reactor interposed therebetween. The converter unit is subjected to PWM current control by a control unit including a one-chip microcomputer, and the inverter unit is subjected to frequency control also by PWM control of a control unit including a microcomputer. The microcomputer of the converter unit receives a zero-cross signal from a power supply for effecting synchronization using the same. Methods of synchronization with a power supply are suggested in JP-A-62-171470, JP-A-62-290359, JP-A-63-7165 and JP-A-63-7166.
The disadvantages of the conventional methods, however, are that in the case where the power synchronization fails to be effected with high accuracy, the DC output current of the converter unit develops a ripple about 6 or 12 times as large as the power frequency, and that if the inductance of the DC reactor is small, the particular ripple is applied to an induction motor constituting a load through the inverter unit, thereby undesirably causing a torque ripple in the motor. A torque ripple, which is a rotational pulsation, causes a vibration or noise in a mechanical system and may damage a joint or the like parts. Also, an elevator which is driven by an induction motor may become uncomfortable to ride in. This problem is apparently avoided in the conventional methods by setting the inductance of the D reactor at a sufficiently high level. In spite of this, when the whole system is large in capacity, the volume and cost of the DC reactor are increased unavoidably. In a large-capacity system, therefore, it is necessary to reduce both the volume and cost of the DC reactor thereof.
A technique for improving this point has been suggested in Article No. 1483 at the 1987 National Conference of the Institute of Electrical Engineers of Japan. This suggestion was to switch the inverter pulse distribution in accordance with the DC power ripple in controlling a current-type inverter system. The output current of the inverter, however, is not necessarily in sinusoidal form but contains many high harmonic components therein. The torque ripple is thus not reduced sufficiently.
JP-A-62-163577, on the other hand, proposes a system for matching the sinusoidal characteristic of the output of a current-type inverter system with the effective utilization of the output current, wherein a predetermined time interval divided proportionately to the instant values of the sinusoidal waves of three phases, respectively, at each sampling. This prior art method has yet to solve the problems of which high harmonics contained in the output current are to be removed selectively and, for this purpose, how the width of the PWM pulse is to be calculated.
As described above, the conventional methods fail to pay sufficient attention to reducing the inductance of a DC reactor or high harmonic components of the output of a current-type inverter system in full consideration of the control of the inverter unit thereof. It is, therefore, difficult to reduce the size of the DC reactor at a reduced cost or reduce the high harmonic output components.